ACOUSTIC EMISSION INDUCED BY PORE-PRESSURE PULSES IN SANDSTONE SAMPLES

An understanding of microseismicity induced by pore-pressure changes is important for applications in geothermal and hydrocarbon reservoirs as well as for CO₂ sequestrations. We studied the spatiotemporal distribution of microseismicity (or acoustic emission in the laboratory) in a water saturated Flechtingen sandstone as a function of triaxial stress conditions and pore-pressure changes.

We present two different types of experiments: In the first experiment triaxial load was applied to a previously unstressed sample in one loading step and then the deformation was kept constant while pore pressure pulses with increasing magnitude were applied at the bottom end of the sample. In the second experiment we applied additional axial stress between the pore pressure pulses. This results in a highly fractured rock in the later stage of the experiment. For our analysis we assumed that acoustic events are triggered by the pore pressure increase. To estimate pore-pressure changes in the sample, we used an analytical solution of the linear 1D diffusion equation.

The theoretical analysis of the spatiotemporal distribution suggests that for initially insignificantly stressed samples the acoustic events were triggered by the diffusion of a critical pore-pressure level through the sample. The critical level is controlled by the applied pore pressure of the previous cycle according to an apparent Kaiser effect in terms of pore pressure. This memory effect of the rock vanished if additional axial stress was applied to the sample before the next injection cycle. For the experiment with constant deformation the apparent Kaiser effect can be explained by the change of the stress field in the sample. Here the assumption of one failure envelope for all cycles is possible. Whereas for the sample which was reloaded, the analysis yields different failure envelopes for the consecutive cycles, this means that the reloading changes not only the stress field in the sample but also the strength of the rock itself. The behaviour of a highly fractured rock in the final stage of the failure experiments was different. During the formation of the final sample-scale fracture, the spatiotemporal distribution of acoustic emission was more likely controlled by propagation of the fracture than by diffusion of a critical pore pressure level.